Abstract
In this paper we discuss and confront recent results on metallicity variations in the local interstellar medium, obtained from observations of H
ii
regions and neutral clouds of the Galactic ...thin disk, and compare them with recent high-quality metallicity determinations of other tracers of the chemical composition of the interstellar medium as B-type stars, classical Cepheids, and young clusters. We find that the metallicity variations obtained for these last kinds of objects are consistent with each other and with that obtained for H
ii
regions but significantly smaller than those obtained for neutral clouds. We also discuss the presence of a large population of low-metallicity clouds as the possible origin for large metallicity variations in the local Galactic thin disk. We find that such a hypothesis does not seem compatible with: (a) what is predicted by theoretical studies of gas mixing in galactic disks, and (b) the models and observations on the metallicity of high-velocity clouds and their evolution as they mix with the surrounding medium in their fall onto the Galactic plane. We conclude that most of the evidence favors that the chemical composition of the interstellar medium in the solar neighborhood is highly homogeneous.
ABSTRACT
We present a reassessment of the radial abundance gradients of He, C, N, O, Ne, S, Cl, and Ar in the Milky Way using the deep optical spectra of 42 H ii regions presented in Arellano-Córdova ...et al. (2020, 2021) and Méndez-Delgado et al. (2020) exploring the impact of: (1) new distance determinations based on Gaia EDR3 parallaxes and (2) the use of Peimbert’s temperature fluctuations paradigm (t2 > 0) for deriving ionic abundances. We find that distances based on Gaia EDR3 data are more consistent with kinematic ones based on Galactic rotation curves calibrated with radio parallaxes, which give less dispersion and uncertainties than those calibrated with spectrophotometric stellar distances. The distances based on the Gaia parallaxes – DR2 or EDR3 – eliminate the internal flattening observed in previous determinations of the Galactic gradients at smaller distances than ∼7 kpc. Abundances and gradients determined assuming t2 > 0 – not only for O but also for the rest of elements – are not affected by the abundance discrepancy problem and give elemental abundances much consistent with the solar ones for most elements. We find that our radial abundance gradient of He is consistent with the most accurate estimates of the primordial He abundance. We do not find evidence of azimuthal variations in the chemical abundances of our sample. Moreover, the small dispersion in the O gradient – indicator of metallicity in photoionized regions – indicate that the gas of the H ii regions is well mixed in the sampled areas of the Galaxy.
ABSTRACT
We present optical spectra of nine Galactic H ii regions observed with the 10.4 m Gran Telescopio Canarias telescope and located at Galactocentric distances (RG) from 4 to 8 kpc. The ...distances of the objects have been revised using Gaia DR2 parallaxes. We determine the electron temperature for all the nebulae, which allows a precise computation of their ionic abundances. We have included published data of an additional sample of Galactic H ii regions, providing a final data set of 42 objects. The shape of the radial gradients of O/H and N/H is linear and constant, discarding any substantial change of the slope, at least for RG between 4 and 17 kpc. The small dispersion of the O/H and N/H values with respect to the computed gradients imply the absence of significant azimuthal variations of the chemical abundances, at least in the quadrant covered by our observations. We find an almost flat N/O versus O/H diagram relation. This result is not observed in other nearby spiral galaxies except M31. Finally, we compare our computed gradients with those obtained using far-infrared (FIR) spectra. We confirm the significant offset in the N/O distribution between the optical and FIR observations. Possible explanations involve ionization correction factors and the strong dependence on density of the abundance determinations based on FIR lines.
Abstract
We obtained new quantitative determinations of the nitrogen abundance and a consistent relation between the nitrogen and oxygen abundances for a sample of Seyfert 2 galaxies located at ...redshift z < 0.1. We carried out this analysis using the cloudy code to build detailed photoionization models. We were able to reproduce observed optical narrow emission line intensities for 44 sources compiled from the literature. Our results show that Seyfert 2 nuclei have nitrogen abundances ranging from ∼0.3 to ∼7.5 times the solar value. We derived the relation log (N/H) = 1.05( ± 0.09) × log (O/H) − 0.35( ± 0.33). Results for N/O versus O/H abundance ratios derived for Seyfert 2 galaxies are in consonance with those recently derived for a sample of extragalactic disc H ii regions with high metallicity.
ABSTRACT
We use a sample of 154 observations of 124 H ii regions that have measurements of both TeO iii and TeN ii, compiled from the literature, to explore the behaviour of the TeO iii–TeN ii ...temperature relation. We confirm that the relation depends on the degree of ionization and present a new set of relations for two different ranges of this parameter. We study the effects introduced by our temperature relations and four other available relations in the calculation of oxygen and nitrogen abundances. We find that our relations improve slightly on the results obtained with the previous ones. We also use a sample of 26 deep, high-resolution spectra to estimate the contribution of blending to the intensity of the temperature-sensitive line O iii λ4363, and we derive a relation to correct TeO iii for this effect. With our sample of 154 spectra, we analyse the reliability of the R, S, O3N2, N2, ONS, and C strong-line methods by comparing the metallicity obtained with these methods with the one implied by the direct method. We find that the strong-line methods introduce differences that reach ∼0.2 dex or more, and that these differences depend on O/H, N/O, and the degree of ionization.
ABSTRACT
We present a reassessment of the radial abundance gradients of C, N, O, Ne, S, Cl, and Ar in the Milky Way using deep spectra of 33 H ii regions gathered from the literature, covering ...Galactocentric distances from 6 to 17 kpc. The distances of the objects have been revised using Gaia parallaxes. We recalculate the physical conditions and ionic abundances in an homogeneous way using updated atomic data. All the objects have direct determination of the electron temperature, permitting to derive their precise ionic abundances. We analyse and compare different ICF schemes for each element in order to obtain the most confident total abundances. Due to the revised distances, our results do not support previous claims about a possible flattening of the O/H gradient in the inner Galactic disc. We find that the Galactic N/O gradient is rather flat, in contrast to what has been found in other spiral galaxies. The slope of the gradients of some elements is sensitive to the ICF scheme used, especially in the case of Ne. The dispersion around the fit for the gradients of C, N, O, S, Cl, and Ar is of the order of the typical uncertainties in the determination of the abundances, implying the absence of significant inhomogeneities in the chemical composition of the ionized gas phase of the ISM. We find flat gradients of log(S/O) and log(Cl/O) and very shallow or flat ones for log(Ne/O) and log(Ar/O), consistent with a lockstep evolution of Ne, S, Cl, and Ar with respect to O.
ABSTRACT
We present the analysis of physical conditions, chemical composition, and kinematic properties of two bow shocks – HH 529 II and HH 529 III – of the fully photoionized Herbig–Haro object ...HH 529 in the Orion Nebula. The data were obtained with the Ultraviolet and Visual Echelle Spectrograph at the 8.2m Very Large Telescope and 20 yr of Hubble Space Telescope imaging. We separate the emission of the high-velocity components of HH 529 II and III from the nebular one, determining ne and Te in all components through multiple diagnostics, including some based on recombination lines (RLs). We derive ionic abundances of several ions, based on collisionally excited lines and RLs. We find a good agreement between the predictions of the temperature fluctuation paradigm (t2) and the abundance discrepancy factor (ADF) in the main emission of the Orion Nebula. However, t2 cannot account for the higher ADF found in HH 529 II and III. We estimate 6 per cent of Fe in the gas phase of the Orion Nebula, while this value increases to 14 per cent in HH 529 II and between 10 and 25 per cent in HH 529 III. We find that such increase is probably due to the destruction of dust grains in the bow shocks. We find an overabundance of C, O, Ne, S, Cl, and Ar of about 0.1 dex in HH 529 II and III that might be related to the inclusion of H-deficient material from the source of the HH 529 flow. We determine the proper motions of HH 529 finding multiple discrete features. We estimate a flow angle with respect to the sky plane of 58° ± 4° for HH 529.
ABSTRACT
We present a first study based on the analysis of the DEep Spectra of Ionized REgions Data base (DESIRED). This is a compilation of 190 high signal-to-noise ratio optical spectra of ...$\mathrm{H\, \scriptstyle II}$ regions and other photoionized nebulae, mostly observed with 8–10 m telescopes and containing ∼29 380 emission lines. We find that the electron density –ne– of the objects is underestimated when $\mathrm{S\, \scriptstyle II}$ λ6731/λ6716 and/or $\mathrm{O\, \scriptstyle II}$ λ3726/λ3729 are the only density indicators available. This is produced by the non-linear density dependence of the indicators in the presence of density inhomogeneities. The average underestimate is ∼300 cm−3 in extragalactic $\mathrm{H\, \scriptstyle II}$ regions, introducing systematic overestimates of Te($\mathrm{O\, \scriptstyle II}$) and Te($\mathrm{S\, \scriptstyle II}$) compared to Te($\mathrm{N\, \scriptstyle II}$). The high-sensitivity of $\mathrm{O\, \scriptstyle II}$ λλ7319 + 20 + 30 + 31/λλ3726 + 29 and $\mathrm{S\, \scriptstyle II}$ λλ4069 + 76/λλ6716 + 31 to density makes them more suitable for the diagnosis of the presence of high-density clumps. If Te($\mathrm{N\, \scriptstyle II}$) is adopted, the density underestimate has a small impact in the ionic abundances derived from optical spectra, being limited to up to ∼0.1 dex when auroral $\mathrm{S\, \scriptstyle II}$ and/or $\mathrm{O\, \scriptstyle II}$ lines are used. However, these density effects are critical for the analysis of infrared fine structure lines, such as those observed by the JWST in local star forming regions, implying strong underestimates of the ionic abundances. We present temperature relations between Te($\mathrm{O\, \scriptstyle III}$), Te($\mathrm{Ar\, \scriptstyle III}$), Te($\mathrm{S\, \scriptstyle III}$), and Te($\mathrm{N\, \scriptstyle II}$) for the extragalactic $\mathrm{H\, \scriptstyle II}$ regions. We confirm a non-linear dependence between Te($\mathrm{O\, \scriptstyle III}$) and Te($\mathrm{N\, \scriptstyle II}$) due to a more rapid increase of Te($\mathrm{O\, \scriptstyle III}$) at lower metallicities.
We study the sensitivity of the methods available for abundance determinations in H ii regions to potential observational problems. We compare the dispersions they introduce around the oxygen and ...nitrogen abundance gradients when applied to five different sets of spectra of H ii regions in the galaxy M81. Our sample contains 116 H ii regions with galactocentric distances of 3 to 33 kpc, including 48 regions observed by us with the OSIRIS long-slit spectrograph at the 10.4-m Gran Telescopio Canarias telescope. The direct method can be applied to 31 regions, where we can get estimates of the electron temperature. The different methods imply oxygen abundance gradients with slopes of -0.010 to -0.002 dex kpc..., and dispersions in the range 0.06-0.25 dex. The direct method produces the shallowest slope and the largest dispersion, illustrating the difficulty of obtaining good estimates of the electron temperature. Three of the strong-line methods, C, ONS, and N2, are remarkably robust, with dispersions of ~0.06 dex, and slopes in the range -0.008 to -006 dex kpc... The robustness of each method can be directly related to its sensitivity to the line intensity ratios that are more difficult to measure properly. Since the results of the N2 method depend strongly on the N/O abundance ratio and on the ionization parameter, we recommend the use of the C and ONS methods when no temperature estimates are available or when they have poor quality, although the behaviour of these methods when confronted with regions that have different properties and different values of N/O should be explored. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
We analyze the physical conditions, chemical composition, and other properties of the photoionized Herbig–Haro (HH) object HH 204 through Very Large Telescope echelle spectroscopy and Hubble ...Space Telescope imaging. We kinematically isolate the high-velocity emission of HH 204 from the emission of the background nebula and study the subarcsecond distribution of physical conditions and ionic abundances across the HH object. We find that low- and intermediate-ionization emission arises exclusively from gas at photoionization equilibrium temperatures, whereas the weak high-ionization emission from HH 204 shows a significant contribution from higher-temperature shock-excited gas. We separately derive the ionic abundances of HH 204, the emission of the Orion Nebula, and the fainter diffuse blue layer. In HH 204, the O
+
abundance determined from collisional excited lines matches the one based on recombination lines, while the O
2+
abundance is very low, so that the oxygen abundance discrepancy is zero. The ionic abundances of Ni and Fe in HH 204 have similar ionization and depletion patterns, with total abundances that are a factor of 3.5 higher than in the rest of the Orion Nebula due to dust destruction in the bow shock. We show that a failure to resolve the kinematic components in our spectra would lead to significant error in the determination of chemical abundances (for instance, a 40% underestimate of O), mainly due to incorrect estimation of the electron density.
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